1. Field of the Invention
The present invention relates to a flat, built-in resistors and capacitors for a printed circuit board, and particularly, to a flat resistor and capacitor simultaneously formed on a substrate by etching.
2. Description of Related Art
Many circuits are on or inside a multi-layered printed circuit board. The circuits serve as an electrical connection or distribute signals among the electronic components. However. to respond to the fast development in information technology and the great increase in functions, more components are needed and higher density of components is needed on printed circuit boards. Under these circumstances, some problems occur.
1. As the quantity of electronic components increases, the distance between adjacent components is dramatically reduced so as to promote the opportunity for radio disturbance among them. thus adversely affecting the stability of the operating circuit.
2. Due to the greater quantity of electronic components, different routes are needed for transmission of the signals among components (for example, through connected holes for electrical connection or conveyance of signals), thus impedance mismatch and chaotic signals in the circuit are added.
3. Due to the greater quantity of electronic components, the excellent production yield of the circuit boards is reduced resulting in increased production cost.
4. The greater quantity of electronic components on a circuit board necessitates an increase in the total area of the circuit board.
To avoid the foregoing problems in high-density printed circuit boards, OHMEGA company has developed a new technology in terms of built-in resistors to replace the resistors assembled on the surface of the printed circuit board. The principle of this technology is based on the following formula: EQU R=(.rho./t).times.L/W
wherein R=resistance, .rho.=resistance coefficient, t=thickness, and W=width. PA1 wherein the .di-elect cons..sub.r is the dielectric coefficient, A is the area for embedding the resistors, t is the thickness (mil) of the high-dielectric substrate, and k is a constant. PA1 wherein the .rho. is the resistance coefficient of the layer of high-dielectric material, and L, W, and T respectively are the length, width and thickness of the layer of high-dielectric material.
According to the foregoing formula, the value of R can be adjusted by changing the length L, the width W or both. Therefore, OHMEGA produces resistors having the desired resistance by also adjusting the values of the resistance coefficient .rho. and the thickness t.
Additionally, in current multi-layered printed circuit boards, the resistors are formed inside the circuit board to form built-in resistors to effectively replace the resistors formed on the surface of the circuit board so as to reduce the area occupied by components on the surface of the circuit board.
In addition to built-in resistors capacitors are often formed in many ways. in current multi-layered printed circuit boards, two layers (for example, VCC and GND) having different potential are separated by a small distance to produce an additional capacitor between the copper surface of these two layers and are used to adjust the voltage. The formula for the foregoing capacitor is: EQU C(capacitance)=.di-elect cons..sub.r (dielectric value).times.A(area)/d(separation)
wherein the .di-elect cons..sub.r is dependent on the material used and thus the capacitance needed can be controlled by changing the area A and the separation d.
In view of the foregoing, components, such as resistors and the capacitors, can be practically formed inside the circuit board to free space on the surface of the circuit board. However, the foregoing built-in and embedded resistors and capacitors are formed by independent techniques rather than being formed simultaneously with the board, such that the manufacturing steps are complicated. In addition, the resistors and the capacitors are formed on a different substrate resulting in the waste of material and increasing the manufacturing cost. Therefore, a need still exists to provide a new flat, built-in resistor and capacitor for a printed circuit board.